This invention relates to transmitters of the type used in the process control industry. More specifically, this invention relates to a process transmitter having a communications circuit for transmitting information over a process link.
Process transmitters are used to monitor industrial processes and responsively transmit information relating to the industrial process to a process device, such as a control room, a controller, another process transmitter, or a process control such as a valve. For example, various fluid process variables such as level, flow, mass, pressure, and temperature are monitored and transmitted to the process device at facilities such as oil refineries, wood pulp processing plants, or chemical plants. Process transmitters are typically contained in an explosion-proof housing and are frequently located to monitor an industrial process in areas that are difficult to physically access with nearby hazards. For example, a process transmitter may be located in an area such as on top of a many thousand gallon tank or in a confined space. Hazards in the vicinity of process transmitters include machinery, chemicals, radiation, temperature extremes, and high voltage.
The process transmitter communicates an information signal to a process device over process link wires. In some applications, the wires coupling the process transmitter to the process device are configured with dedicated wires such as a process loop, and, in other applications, the wires are configured as common wires such as a process bus. The information signal includes signals representing one or more process parameters and signals representing process transmitter information such as configuration and status. These signals may be digital signals, analog signals, or a combination of digital and analog signals configured according to a process communications protocol. The information signal is transmitted over the process link to the process device where the information signal is interpreted and decisions are made concerning the process or the process transmitter based upon the information signal. In most previous transmitters, the process link requires power before the process transmitter is capable of generating an information signal.
Before a process transmitter is installed to monitor an industrial process, the process transmitter is connected to a communicator such as a Rosemount.RTM. Model 268 HART.RTM. Smart Family interface for diagnostic testing and setup. Diagnostic testing is performed to validate that the process transmitter is operating according to the manufacturer's specifications. Process transmitter setup, also known as commissioning, is performed to program the transmitter with information that relates to the particular process to be monitored such as transmitter configuration, transmitter address, installation temperature, and transmitter height. Previous process transmitters require connection to a powered process link, which comprises a power supply such as a 10-30V DC battery and a load resistor such as a 250.OMEGA. or greater resistor, to measure current flowing through the process link. Connection to the power supply and load resistor can make commissioning of a process transmitter inconvenient and time consuming.
Field diagnostic testing and set-up changes to installed process transmitters are sometimes difficult and dangerous because process transmitters are often located in areas that are difficult to access and potentially hazardous. As discussed above, some previous process transmitters require connection to a powered process link comprising a power supply and load resistor. Physically transporting a power supply and a load resistor into a difficult to access and potentially hazardous area can make field diagnostic testing and set-up changes to an installed process transmitter inconvenient, time consuming, and dangerous.
When the power on the process link becomes insufficient to support communications, previous current sink process transmitters are unable to communicate over the process link to a process device to report the status of the transmitter or process. When the transmitter is unable to communicate, a service person may be dispatched to physically check the transmitter and provide alternate means for reporting process conditions. In some situations, before a service person is permitted access to the transmitter, the process may have to be halted or delayed to provide a safe environment for the service person. Halting or delaying the process often results in process complications such as unacceptable process product, lost process production time, and undergoing start-up procedures.
For the foregoing reasons, a process transmitter is needed that can be commissioned and, while installed, undergo diagnostic testing and set-up all without the inconvenience of providing power on the process link. A process transmitter is also needed that, once installed, has the capability to communicate when there is inadequate power on the process link.